Light and heavy reflux streams in solvent extraction of aromatics

ABSTRACT

PROCESS FOR SEPARATING AND RECOVERING AROMATIC HYDROCARBONS, SUCH AS BENZENE, FROM A HYDROCARBON FEED MIXTURE VIA AN IMPROVED SOLVENT EXTRACTION TECHNIQUE. THE PREFERRED SOLVENT COMPRISES SULFOLANE AND THE EXTRACTION   ZONE IS REFLUXED WITH A SPECIFIC NON-AROMATIC HYDROCARBON STREAM.

EXTRCTION 0F AROMATICS Filed Oct. 28, 1968 Jan. 26, 1971 mm\ A Anw AA .SSSQAA A m fw/s a A Z. n A.. i /A ,-A. Wawy?" A V 7 .A QA .t A A A A /M .WA At A, Amm w AA A m\\ vw\ A A A AA 0| \.\\A A A Wr /A.N\ /bmm A A t A A A A APCM A A \V\ A A Q f2.5@ A A. nm AA SA AA A ,.A AAM A 3,558,480 LIGHT AND HEAVY REFLUX STREAMS IN SOLVENT EXTRACTION F AROMATICS Donald B. Broughton, Evanston, Ill., assignor to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Oct. 28, 1968, Ser. No. 771,246 Int. Cl. 010g 21 /22 U.S. Cl. 208-318 2 Claims ABSTRACT OF THE DISCLOSURE Process for separating and recovering aromatic hydrocarbons, such as benzene, from a hydrocarbon feed mixture via an improved solvent extraction technique. The preferred solvent comprises sulfolane and the extraction zone is refluxed with a specific non-aromatic hydrocarbon stream.

BACKGROUND OF THE INVENTION This invention relates to a separation process. It also relates to a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures utilizing a selective solvent.

The technique of aromatic hydrocarbon separation by solvent extraction is Well known in the prior art. Generally, according to one well known procedure, a mixture of hydrocarbons comprising aromatic and non-aromatic hydrocarbons is introduced into an extraction column at an intermediate point thereof and contacted with a solvent which is selective for aromatic hydrocarbons. The separating conditions maintained in the extraction column are suicient to produce a rainate phase comprising substantially all of the non-aromatic hydrocarbons in the feedstock and an extract phase comprising the aromatic hydrocarbon components of the feedstock dissolved in the selective solvent. Typically, the non-aromatic hydrocarbon raffinate phase is recovered and utilized in, for example, gasoline blending. Ihe extract phase containing the aromatic hydrocarbons is passed subsequently through various separation means for the recovery therefrom of the aromatic hydrocarbons in high concentration and high purity. Lean solvent, generally suitable for reuse in the extraction zone, is also recovered in the process of segregating and recovering the aromatic hydrocarbons.

One feature of the prior art process is the freeing of the extract phase of substantially all of the non-aromatic components dissolved by or carried by the selective solvent. Typically, this is accomplished by introducing a non-aromatic hydrocarbon reflux stream comprising, for example, naphthenes and parailins, into the extraction zone at a point intermediate between the feed introduction point and the extract phase withdrawal point. This reflux phase is desirably more voltatile in the presence of the selective solvent than any of the aromatic components of the feedstock and, in effect, acts as a displacing agent for the higher boiling non-aromatic hydrocarbons out of the extract phase and into the raffinate stream Within the extraction zone.

However, the forced migration of the non-aromatic hydrocarbon components from the extract phase to the raflinate phase is only partially successful since the prior art reflux streams also comprise non-aromatic hydrocarbons having such a high boiling point that these are not easily separable from aromatic hydrocarbons in the subsequent extractive stripping operation. In effect, therefore, the purity of the ultimate aromatic hydrocarbon product stream is dependent to a large extent upon the ability of the extraction zone to separate both the relatively heavy and the relatively light non-aromatic hydrol United States Patent O carbon components. It would be desirable if a reflux stream could be fed to the extraction zone substantially free of relatively heavy non-aromatic components.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a separation process.

It is another object of this invention to provide a solvent extraction process for the recovery of aromatic hydrocarbons from hydrocarbon mixtures.

It is still another object of this invention to provide an improved process for the separation and recovery of aromatic hydrocarbons in high purity via a solvent extraction technique in a facile and economical manner.

Therefore, the present invention provides a process for separating aromatic hydrocarbons from a feed mixture containing aromatic and non-aromatic hydrocarbons which comprises the steps of: (a) introducing said feed mixture into a solvent extraction zone in contact with lean solvent and a hereinafter specified reflux stream under conditions sufficient to produce a rainate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing contaminating quantities of non-aromatic hydrocarbons; (b) passing said extract stream into a distillation zone maintained under distillation conditions; (c) withdrawing from said distillation zone an overhead vapor fraction containing solvent and said non-aromatic hydrocarbon contaminants, and a bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein; (d) partially condensing said overhead vapor fraction to produce a rst liquid fraction comprising relatively heavy non-aromatic hydrocarbons, and a second vapor fraction; (e) totally condensing said second vapor stream thereby producing a second liquid fraction comprising relatively light nonaromatic hydrocarbons; (f) passing at least a portion of said second liquid fraction into said extraction zone as the specified reflux stream; and (g) recovering aromatic hydrocarbons from said bottoms fraction.

Another embodiment of this invention is the process hereinabove wherein said first liquid fraction is returned to said extraction zone.

In essence, therefore, the present invention embodies the concepts of solvent extraction, distillation, and partial condensation of the non-aromatic hydrocarbon vapor stream in order to produce a tailor made relatively light hydrocarbon material which is utilized as the displacing agent in the extraction zone..

DETAILED DESCRIPTION OF THE INVENTION As a broad general class, suitable feedstocks for the satisfactory practice of this invention include iluid mixtures having a suliciently high concentration of aromatic hydrocarbons to economically justify recover of these aromatic hydrocarbons as a separate product stream. The present invention is particularly applicable to hydrocarbon feed mixtures which contain at least 25% by Weight aromatic hydrocarbons. A suitable carbon number range for the feedstock is from about 6` carbon atoms per molecule to about 20 carbon atoms per molecule and, preferably, from about 6` to 10 carbon atoms per molecule. One source of feedstock is the debutanized reactor effluent from a conventional catalytic reforming process unit. Another `source of satisfactory feedstock is the liquid by-product from a pyrolysis gasoline unit which has been hydrotreated to saturate olens and diolens, thereby producing an aromatic hydrocarbon concentrate suitable for the solvent extraction technique described herein. Typically, the feedstock from a catalytic reforming process unit contains single ring 3 aromatic hydrocarbons comprising a wide boiling mixture of benzene, toluene, and xylenes. These single ring aromatic hydrocarbons are mixed with the corresponding parains and naphthenes which have been produced from such a catalytic reforming unit.

The preferred solvent which may be utilized in the present invention is a solvent of the sulfolane-type. This solvent is well known to those skilled in the art and, typically, possesses a live membered ring containing one (l) atom of sulfur and four (4) atoms of carbon with two (2) oxygen atoms bonded to the sulfur atom of the ring. Since this solvent is an article of commerce and is well known to those versed in the solvent extraction art, greater detail thereof need not be presented at this time.

yOther solvents which may be included and may satisfactorily be used in the practices of this invention are the sulfolenes, such as 2-sulfolene and 3-sulfolene. Still other typical solvents which have a high selectivity for separating aromatic hydrocarbons from non-aromatic hydrocarbons and which may be processed within the cope of the present invention are Z-methyl-sulfolane, 2-4, dimethylsulfolane, methyl-Z-sulfonylether, n-aryl- 3-sulfonylamine, 2-sulfonylacetate, diethleneglycol, various polyethleneglycols, dipropyleneglycol, various polypropyleneglycols, dimethylsulfoxide, N-methylpyrrolidone various mixtures of the above, and the like. The specifically preferred solvent for use in the practice of the invention is sulfolane.

The aromatic selectivity of the solvent can usually be further enhanced by the addition of water to the solvent. Preferably, the solvents utilized in the practice of this invention contain small quantities of water in order to increase the selectivity of the overall solvent phase for aromatic hydrocarbon over non-aromatic hydrocarbons without reducing substantially the solubility of the solvent phase for aromatic hydrocarbon. The presence of water in the solvent composition further provides a relatively volatile material therein which is distilled from the solvent in the extractor stripper, more fully discussed hereinafter, to vaporize the last traces of non-aromatic hydrocarbons from the solvent stream by steam distillation. The composition of the present invention preferably contains from about 0.5% to about 20% by weight water and preferably from about by Weight depending upon the particular solvent utilized and the process conditions at which the extraction zones and extractive stripper are operated.

The extraction zone of the present invention is operated at elevated temperature and sufficiently elevated pressure to maintain the feedstock, the solvent and reflux streams in liquid phase. Typically, suitable temperatures when using sulfolane as the solvent are within a range from about 80 F. to about 400 F. and, preferably, from about 175 F. to about 300 F. Similarly, suitable pressures are generally within the range from about atmospheric pressure up to about 400 p.s.i.g. and, preferably, from 50 to 150 p.s.i.g.

It is noted from the description of the present invention thus far that one of the critical problems to which this invention is directed is the displacement of non-aromatic hydrocarbons from the extract phase at the lower end of the extraction zone by utilizing the technique of a specific non-aromatic hydrocarbon containing reux at that point. It is distinctly preferred that this reux stream comprise relatively light non-aromatic hydrocarbons and may contain significant quantities of aromatic hydrocarbons, e.g. from 30% to 60% by weight. In other words, the preferred practice of this invention is to operate a second distillation column in such a manner that a reflux stream is obtained which comprises a concentrate of the lightest non-aromatic hydrocarbon components which were left in the extract phase from the extraction zone. In conjunction therewith, the present invention provides operating flexibility wherein relatively heavy non-aromatic hydrocarbons may also be obtained either as a separate product stream and/or utilized as an additional non-aromatic hydrocarbon reflux in the extraction zone, details of which will be more fully discussed hereinbelow. The ability to tailor the physical characteristics of the reux stream to the needs within the extraction zone for a displacing agent permits increased eliciences to be obtained in the solvent extraction technique whereby increased yields and purity of aromatic hydrocarbons will result therefrom.

The amount of reflux introduced into the lower end of the extraction zone may be varied considerably depending upon the degree to which non-aromatic hydrocarbons are being rejected in the extraction zone. It is preferred that the relatively light non-aromatic hydrocarbon reflux introduced into the lower end of the extraction zone be at least 10% by volume of the extract phase leaving the extraction zone in order to effectively displace non-aromatic hydrocarbons from the extract phase into the raffinate phase.

The first distillation column which is utilized in the processing train of the present invention is operated at moderate pressures and substantially high reboiler temperatures to force all of the non-aromatic hydrocarbon material and a portion of the aromatic hydrocarbons, water, and solvent out of the bottom portion of this column. It is preferred that the rst distillation column be operated substantially as a stripper column; that is, the extract phase should be fed into the rst distillation column at the upper portion thereof in order to provide maximum stripping to take place in the column. In order for extremely high purity aromatic hydrocarbons to be ultimately obtained, it is essential that this first distillation column be operated in a manner to prevent all except minute traces of nonaromatic hydrocarbons from being withdrawn from the bottom of this column.

Typically, the extractive stripped will be maintained under pressures from atmospheric to about p.s.i.g.; although, generally, the top of the stripper column is maintained at from 10 to about 20 p.s.i.g. The reboiler temperature maintained in the extractive stripper is, of course, dependent upon the composition of the feedstock and the solvent. The column is arranged to take only an overhead fraction and a bottoms fraction as separate product streams. Those skilled in the art, from a knowledge of the prior art, are well versed in the operation of a stripper column of this type and, therefore, additional details thereof need not be presented here.

The aromatic and solvent recovery column is operated at low pressures and sufficiently high temperatures to distill the aromatic hydrocarbons overhead as a distillate fraction, thereby producing a bottoms fraction comprising lean solvent which is generally suitable for reuse in the solvent extraction zone. Again, the choice of operating conditions depends upon the feedstock characteristics and the composition of the solvent. Preferably, the top of the aromatic recovery column is operated at about 100 to 400 mm. of mercury absolute. These sub-atmospheric pressures must be employed in order to maintain a sufficiently low reboiler temperature to avoid thermal decomposition of the solvent, particularly when the solvent is of the sulfolane-type. Preferably, the reboiler temperature should be maintained below about 360 F. when using saturated sulfolane as the solvent.

Returning now to the operation of the extractive stripper column: the overhead stream from the extractive stripper column is in the vapor state and comprises primarily nonaromatic hydrocarbons of the naphthene and/or parafiin type. According to the practice of this invention, the overhead vapor stream for the extractive stripper column is partially condensed in a heat exchange condenser thereby producing a liquid fraction which is enriched in relatively heavy non-aromatic hydrocarbons. This enriched stream is preferably returned at least in part to the extraction zone in admixture with the feed stream. Alternatively, this enriched liquid stream may be introduced into the solvent extraction zone at a locus between the feed locus and the withdrawal point of the raffinate phase, if desired. Still further, the enriched liquid stream may be removed from the process as a separate product stream for utilization in, for example, gasoline blending. The vapor phase remaining for the partial condensing operation is then passed into a final condensing zone under conditions suicient to produce a liquid phase comprising relatively light or low boiling non-aromatic hydrocarbons. This latter liquid stream is utilized in the practice of this invention as the reflux stream passing into the lower end of a vertically disposed solvent extraction zone in the manner previously mentioned.

The bottoms material from the extractive stripper is withdrawn and passed into a solvent recovery column which is operated at low pressures, as previously mentioned, in order to separate and recover the aromatic hydrocarbon, e.g. benzene, from the solvent phase and to recover a lean solvent stream suitable for reuse in the extraction zone.

The invention may be further understood with reference to the appended drawing which is a schematic representation of apparatus for practicing one embodiment of the invention.

DESCRIPTION OF THE DRAWING Referring now to the drawing, hydrocarbon feedstock containing aromatic hydrocarbons, such as the desired product from a conventional catalytic reforming unit comprising benzene, toluene, and xylene mixed with corresponding naphthenes and paraffins, enters the system via line 11 wherein it is admixed with a relatively heavy nonaromatic hydrocarbon stream in line 24 from a source hereinafter discussed. The combined feed mixture is introduced via line 12 into extractor 13. Aqueous sulfolane as the lean solvent enters the extractor at an upper portion thereof via line 16 and the relatively light non-aromatic hydrocarbon reflux from a source hereinafter described enters extractor 13 at the lower end thereof via line 28. A raffinate stream is withdrawn from extractor 13 via line 14 as a product stream. An extract phase comprising sulfolane solvent having aromatic hydrocarbons dissolved therein and which is contaminated with minor amounts of non-aromatic hydrocarbons, such as the corresponding naphthenes and parains, is withdrawn from extractor 13 via line 15.

According to the practice of this invention, the extract phase in line 15 is passed into extractive stripper column 17. Additional sulfolane solvent may be added to extractor stripper column 17 as needed from a source not shown iu order to enhance the separation of the non-aromatic hydrocarbons from the solvent containing the aromatic hydrocarbons. Sufficient separation or distillation conditions, as previously mentioned, are maintained in extractor stripper 17 to produce a bottoms fraction comprising solvent having dissolved therein the desired aromatic hydrocarbons, such as benzene, toluene, and xylene, said bottoms being removed from distillation column 17 via line 18.

The distillation conditions in extractor stripper 17 are also sufficient to produce a distillate fraction comprising essentially all of the non-aromatic hydrocarbons and minor amounts of solvent and aromatic hydrocarbons. This distillate fraction is withdrawn from stripper 17 via line 19 and passed into partial condenser 20 which is maintained under conditions sufficient to produce a liquid phase and a vapor phase in separation zone 22. Normally, the conditions maintained in condenser 20 will be sufficient to produce from to 95% liquid in separator 22 depending upon the relative amount of non-aromatic hydrocarbons present in the original feedstock which is condensed, and the vapor is passed via line 21 into separator 22 for separation as aforesaid. The condensed liquid in separator 22 comprises relatively non-aromatic hydrocarbons and is withdrawn via line 24 and passed into admixture with the incoming feed in line 11 as [previously mentioned.

The remaining vapor from separator 22 is withdrawn via line 23 and passed into final condenser 25 for the production of another liquid phase containing relatively light non-aromatic hydrocarbons which are accumulated in accumulator 27. The condensed liquid and vapor passes via line 26 from final condenser 251 into accumulator 27. The liquid phase comprising the desired and tailor made light non-aromatic hydrocarbons is passed via line 28 into the lower end of extractor 13 thereby displacing additional relatively heavy non-aromatic hydrocarbons from the extract phase at the lower end of extractor 13.

Referring now to the solvent and aromatic hydrocarbon stream in line `18: this material is passed into recovery column 29 which is maintained under conditions suicient to separate the aromatic hydrocarbons from the solvent phase. The aromatic hydrocarbons are concentrated as a distillate fraction and removed from recovery column 29 via line 30'. It is to be noted that the material in line 30 contains, for example, benzene, which was originally present in the feed from line 11. The operation of recovery column 29 includes the use of steam stripping on the column in order to remove the nal traces of aromatic hydrocarbons from the solvent. Therefore, the lower end of column 29 contains the desired solvent plus Water in sufficient amounts such that the material withdrawn from column 29 via line 16 comprises lean solvent suitable for reuse in the extratcion zone.

The invention claimed is:

1. Process for separating aromatic hydrocarbons from a feed mixture containing aromatic and non-aromatic hydrocarbons which comprises the steps of:

(a) introducing said feed mixture together with a hereinafter specified heavy reflux stream into the upper portion of a solvent extraction zone and therein contacting the same with a lean solvent under conditions sufficient to produce a raflnate stream and an extract stream comprising solvent having aromatic hydrocarbons dissolved therein and containing contaminating quantities of non-aromatic hydrocarbons;

(b) passing said extract stream into a distillation zone maintained under distillation conditions;

(c) withdrawing from said distillation zone an overhead vapor fraction containing solvent and said nonaromatic hydrocarbon contaminants, and a bottoms fraction comprising solvent having aromatic hydrocarbons dissolved therein;

(d) partially condensing said overhead vapor fraction to produce a first liquid fraction comprising relatively heavy non-aromatic hydrocarbons and a second vapor stream, and commingling at least a portion of said rst liquid fraction with said feed mixture as said heavy reflux stream;

(e) totally condensing said second vapor stream thereby producing a second liquid fraction comprising relatively light non-aromatic hydrocarbons;

I(f) introducing at least a portion of said second liquid fraction into the lower portion of said extraction zone as a light reflux stream therein; and

(g) recovering aromatic hydrocarbons from said bottoms fraction.

2. Process of claim 1 further characterized in that said solvent comprises sulfolane.

References Cited UNITED STATES PATENTS 3,262,875 7/ 1966 Girotti et a1. 208-321 3,325,399 6/ 1967 Cinelli et al 260-674 3,436,435 4/ 1969 Van Tassell 260-674 3,468,792 9/ 1969 Uitti 208-318 'HERBERT LEVINE, Primary Examiner U.S. Cl. X.R. 

